One problem often encountered in the production of natural gas from underground reservoirs is nitrogen contamination. The nitrogen may be naturally occurring and/or may have been injected into the reservoir as part of an enhanced oil recovery (EOR) or enhanced gas recovery (EGR) operation. Natural gases which contain a significant amount of nitrogen may not be saleable, since they do not meet minimum heating value specifications and/or exceed maximum inert content requirements. As a result, the feed gas will generally undergo processing, wherein heavier components such as natural gas liquids are initially removed, and then the remaining stream containing primarily nitrogen and methane, and also possibly containing lower boiling or more volatile components such as helium, hydrogen and/or neon, is separated cryogenically. A common process for separation of nitrogen from natural gas employs a double column distillation cycle, similar to that used for fractionation of air into nitrogen and oxygen.
A problem often encountered in the cryogenic separation of nitrogen and methane is the loss of some methane with the nitrogen overhead from the nitrogen rejection unit. This is especially the case where the nitrogen concentration in the feed is less than about 30 percent. In such situations there is less nitrogen available for reflux and thus the separation of the nitrogen and methane is carried out to a lesser extent than is desirable.
The problem of inadequate nitrogen reflux in a nitrogen rejection unit has been addressed by recirculating some of the nitrogen product from the separation back to the nitrogen rejection unit. Although such a system is effective in upgrading the reflux available for separation, it is disadvantageous because nitrogen which has already been separated from the nitrogen-methane mixture is returned and must be separated a second time.
A recent significant advancement in the cryogenic separation of nitrogen and methane is disclosed and claimed in U.S. Pat. No. 4,664,686-Pahade et al. In this system a stripping column is provided upstream of the nitrogen rejection unit. The stripping column serves to increase the nitrogen content of the feed to the nitrogen rejection unit thus eliminating the need for nitrogen recompression and recirculation. Another advantage of this stripping column process is that a large fraction of the methane is recoverable directly from the stripping column at an elevated pressure thereby reducing subsequent compression requirements. Still another advantage of this process is that tolerance to carbon dioxide presence in the feed is improved.
The stripping column of a nitrogen rejection system may have an optimum operating pressure lower than that of the feed. This reduces the pressure at which the nitrogen rejection unit can operate and thus reduces the potential pressure of its methane product. It would be desirable to have a nitrogen rejection unit which can produce higher pressure methane product and thus reduce product compression requirements.
Accordingly, it is an object of this invention to provide an improved stripping column/nitrogen rejection unit wherein the nitrogen rejection unit operation is at least in part decoupled from the stripping column operation so that methane product from the nitrogen rejection unit may be produced at a higher pressure than would otherwise be possible.